The proposed research would assess the etiology of individual differences in prereading and early reading development, and their covariation with individual differences in attention/hyperactivity. We will conduct a longitudinal study of 608 identical and fraternal twin pairs, beginning at 4 years of age, and continuing through their early reading development at the end of kindergarten, first grade, and second grade. The twin data will be used to explore the contributions of genetic, shared-environmental and non-shared environmental influences to individual differences in important pre-reading skills and attention in preschoolers, as well as genetic and environmental links between preschool behavior and subsequent reading development and attention in the early grades. Measures for the preschool children will include a "dynamic" assessment of their ability to learn sensitivity to phonemes in words, as well as measures of other language skills and attention/hyperactivity that are believed to be important in early reading development. Specific environmental influences will be explored using parent and teacher questionnaires and home observations. We plan calls for combining the data from the proposed U.S. twin study sample and a smaller sample of Australian twins included in a parallel study by these investigators that will use the same measures and sampling procedures. While the statistical power of the U.S. sample will be sufficient to address the central questions regarding individual differences, the combined U.S. and Australian samples will allow the assessment of whether genetic and environmental influences are similar for deviant-group membership (i.e., reading disability, attention deficit disorder) versus individual differences across the normal range. These unique longitudinal data from young twins, including dynamic and static measures of prereading skills, environmental measures, and attention measures, will provide important new evidence regarding the genetic and environmental etiology of individual differences across a critical period of reading development from preschool through second grade.